1. Field of the Invention
Example embodiments of the inventive concepts relate to multi-level nonvolatile memory devices using variable resistive elements.
2. Description of the Related Art
Nonvolatile memory devices using a resistive material may include, for example, phase change random access memories (PRAMs), resistive RAMs (RRAMs), magnetic RAMs (MRAMs) and the like. Dynamic RAMs (DRAMs) or flash memory devices may use charge to store data, while nonvolatile memory devices using a resistive material may store data using a variation in the phase or state of a phase change material (e.g., a chalcogenide alloy (PRAM)), a variation in the resistance of a variable resistive element (e.g., a RRAM), or a variation in the resistance of a magnetic tunnel junction (MTJ) film caused by the magnetized state of a ferromagnetic material (e.g., a MRAM).
A resistive memory cell may include an upper electrode, a lower electrode, and a variable resistive element interposed therebetween. The resistance level of the variable resistive element may vary according to a voltage applied between the upper and lower electrodes. In particular, a filament serving as a current path for a cell may be formed in the variable resistive element. A state where the filament is partially disconnected may be defined as a reset state, a high-resistance state, and/or reset data (data 1). A state where the filament is connected may be defined as a set state, a low-resistance state, and/or set data (data 0). A reset bias voltage, which may have a voltage level at which the filament may be disconnected, may be supplied to write reset data into the resistive memory cells. A set bias voltage, which may have a voltage level at which the filament may be reconnected, may be supplied to write set data into the resistive memory cells. Further, a read bias voltage that has a voltage level at which the state of the filament does not change may be supplied to read the stored data to determine whether the read data is reset data or set data.
Various methods for storing as many bits as possible in a limited wafer space have been developed. For example, one possible way of fabricating an increasing number of nonvolatile memory cells in a limited wafer area is to develop and use more sophisticated lithography methods and apparatuses. Alternatively, the integration degree of a nonvolatile memory device may be increased by storing more than one bit in a memory cell (hereinafter referred to as a multi-level nonvolatile semiconductor memory device).
In particular, in a case of a multi-level nonvolatile memory device using a resistive element, each memory cell may have at least three different resistance levels according to data stored in the memory cell. Recently, methods of accurately writing data in the memory cell have been researched and developed.